1. Field of the Invention
This invention relates to lens systems of short total length (from the first surface of the lens to the focal plane) and more particularly to compact wide angle photographic objectives.
2. Description of the Prior Art
Recently, the trend toward minimizing the bulk and size of the camera has increased demands for compact lenses or objective lenses with shortened total lengths. When the total length of a lens system is reduced to less than one times the focal length, it is particularly desirable to make the front lens group of the lens system of a positive refractive power and the rear lens group of a negative refractive power. Such power distribution has found many uses in long lenses with narrow angular fields of view. However, there are a few examples of the application of this principle to objectives having wide angles at least 60.degree. with large aperture ratios and short total lengths.
The reason for this deficiency is that this type of refractive power distribution increases the residual distortion and astigmatism objectionably, or produces extremely large comas and halo, as the total length decreases, the relative aperture increases and the angular field of view widens.
Japanese Patent Application Publication No. Sho 44-10831 discloses a lens system using this type of power distribution. However, the system described therein exhibits a field angle of 46.degree., that is, in the order of a standard lens. If an attempt is made to increase the angular field, the astigmatic difference will increase. U.S. Pat. No. 3,998,527 (Japanese Patent Application Publication No. Sho 52-48011) discloses a similar lens system. However, here the F number is 1:4.5.